In order to control engine performance electronically, it is necessary to provide a variety of signals to the engine control module. These signals indicate the status of the parameter being detected and to which the control must respond. Among status signals necessary is a signal that is indicative of the vehicle speed.
Transmission manufacturers provide a means of generating a signal which is proportional to the rate of rotation of the drive shaft. This is accomplished by placing a tone wheel, a wheel which has about its circumference lands and valleys of equal width, in line with the power take off shaft. The signal which is indicative of the vehicle shaft rotation is generated by inserting a vehicle speed sensor into a port opposite the face of the tone wheel until the sensor comes in contact with the face of the tone wheel, then backed off a half turn or to an orientation mark. The gap between the land of the tone wheel and the sensor tip is usually approximately 0.050 (fifty thousands) of an inch. As the tone wheel rotates, the lands and valleys alternately pass over the sensor head. Each time a land passes, the gap is approximately fifty thousands of an inch. Each time a valley passes, the gap increases to as much as a quarter of an inch. These changes in gap size change the magnetic field concentration between the sensor head and the tone wheel. This change in magnetic field concentration causes self-induction process to take place within the sensor winding which in turn causes a voltage to appear at the coil's output leads. This voltage is semi-sinusoidal whose frequency and voltage amplitude is proportional to the rotational speed of the tone wheel.
Vehicle speed sensors which use a Hall-Effect device as the signal generating element also have been used. These have the advantage of generating a signal of uniform amplitude over the entire vehicle speed range. These too were applied as described above. Again the transducer body contains a Hall-Effect sensing element which is magnetically biased by a permanent magnet mounted in communication with and immediately behind the sensor. The face of the sensing element is approximately 0.015 inches from the end of the sensor body face. As the transducer is screwed into the transducer port of the transmission, it is driven in until it bottoms out against the tone wheel, and is then backed off until an orientation mark aligns in line with the tone wheel. This orientation is important because the Hall-Effect sensing element is position sensitive. The resulting gap between the tone wheel and the sensing element is usually less than 0.050 inches.
As the tone wheel rotates, the lands and valleys pass over the sensing element which changes the concentration of the magnetic field generated by the biasing magnet. These changes in field concentration cause changes in electrical current flow through the sensing element. These changes in current flow are processed by appropriate circuitry, which processing results in a pulse train of constant amplitude but whose pulse rate varies in direction proportion to the rotation speed of the tone wheel. The duty cycle of the output pulses is proportional to the length of the lands and valleys on the tone wheel. Usually, the lengths of the lands and valleys are equal which results in an essentially square wave output.
In an effort to reduce the weight of vehicles and thereby improve fuel efficiency, some manufacturers are using components of lighter weight materials or reducing the size of components whose material properties cannot change. One manufacturer uses a tone wheel which is half the diameter of a previously used tone wheel at a weight savings of approximately three pounds. This reduced diameter is used in the same transmission housing with the same sensor port as was used with the larger diameter tone wheel. End looking vehicle speed sensors when inserted into the same sensor port could no longer butt up against the smaller diameter tone wheel and therefore cannot be used.